The present invention relates to a process for producing p,p'-biphenol. More particularly, the present invention relates to a process for producing p,p'-biphenol by dealkylating 3,3',5,5'-tetra-t-butyl-4,4'-dihydroxybiphenyl.
It is known that when phenols, in particular alkyl-substituted phenols, are oxidized under certain conditions, biphenols and dipheno-quinone compounds are produced by an oxidative coupling reaction. Among the products obtained, p,p'-biphenol is useful as the starting material for the synthesis of polyesters or polycarbonates, as an intermediate for the production of dyes, pharmaceuticals and agri-chemicals, as the starting material for the production of liquid-crystal polymers, or as a photographic agent. Because of this great variety of applications in which p,p'-biphenol finds its potential use, this compound has drawn much attention from researchers in various fields of industry.
When 3,3',5,5'-tetra-t-butyl-4,4'-dihidoxybiphenyl (hereinafter sometimes abbreviated as TTBP) is dealkylated in the presence of both a solvent and a catalyst to obtain p,p'-biphenol (which is hereinafter sometimes abbreviated as DHBP), isobutene is also produced as a by-product. This isobutene is also important for industrial purposes since it can be polymerized into isooctane and polymeric gasoline or can be used as a starting material for the production of butyl rubber by copolymerization of isoprene and isobutene.
While several methods have heretofore been proposed in the art of producing p,p'-biphenol from 3,3',5,5'-tetra-t-butyl-4,4'-biphenol (hereinafter sometimes abbreviated as TTBP), they all have their own problems as described below.
Japanese Pat. Application (OPI) No. 92332/1980 filed by UCC (the term OPI as used herein means an unexamined published Japanese patent application) discloses a process for producing p,p'-biphenol by debutylating TTBP in the absence of both a solvent and a catalyst but this method is not practical for industrial purposes since it requires temperatures of at least 300.degree. C.
Japanese Pat. Application (OPI) No. 1434/1984 discloses a process for simultaneous production of p,p'-biphenol and meta-t-butylphenol by reacting TTBP and phenol under heating in the presence of an activated clay catalyst so that the t-butyl group of TTBP is transalkylated on meta-position of phenol. This method also is not practical for the purpose of industrial production of p,p'-biphenol from TTBP since the meta-t-butylphenol produced from TTBP together with p,p'-biphenol cannot be utilized effectively.
Japanese Pat. Publication No. 135/1987 discloses a method of producing DHBP by debutylation of TTBP using, as a solvent, diphenyl ether which may optionally contain the butylated diphenyl ether resulting from the debutylation of TTBP. Japanese Pat. Application (OPI) No. 189127/1983 discloses a process for producing p,p'-biphenol in which TTBP is debutylated at 120.degree.-250.degree. C. in the presence of catalyst sulfuric acid or sulfonic acid to produce p,p'-biphenol using, as a solvent, phenol which is optionally substituted by a lower alkyl or alkoxy group, and the resulting p,p'-biphenol is separated by filtration at a temperature between the melting point of the solvent and 80.degree. C. However, in each of these methods, the isobutene resulting from the debutylation of TTBP will be added to the unsubstituted phenol or substituted phenols used as a solvent and this "transalkylation" causes t-butylphenol to be produced in large quantities.
Japanese Pat. Application (OPI) Nos. 23338/1985 and 200935/1986 also disclose a method for producing p,p'-biphenol from TTBP using diphenyl ether as a solvent. But in this method part of the isobutene formed by the debutylation of TTBP is transalkylated on the solvent diphenyl ether to produce t-butyldiphenyl ethers, thereby reducing the recovery of isobutene.
If decane or other materials that have low miscibility with TTBP are used as solvents in the production of DHBP from TTBP, the product DHBP will adhere to the wall or the reaction vessel and give considerable difficulty in handling.
It has also been known that p,p'-biphenol can be produced from 2,6-di-t-butylphenol. The route of synthesis of p,p'-biphenol from 2,6-di-t-butylphenol is expressed by the following reaction schemes (1) and (2): ##STR1## where R is a t-butyl group.
An example of the method of performing the first-stage reaction denoted by scheme (1) is disclosed in Japanese Pat. Application (OPI) No.72131/1980, which describes a process for producing 3,3',5,5'-tetra-t-butyl-4,4-dihydroxybiphenyl from 2,6-di-t-butylphenol by performing this first-stage reaction under predetermined conditions in the presence of an alkali catalyst such as an alkali metal hydroxide. However, it is difficult to selectively obtain 3,3',5,5'-tetra-t-butyl-4,4'-dihydroxybiphenyl in high yield by this process and the resulting 3,3',5,5'-tetra-t-butyl-4,4'-dihydroxybiphenyl may be further oxidized to yield 3,3',5,5'-tetra-t-butyl-4,4'-diphenoquinone as a by-product. This by-product 3,3',5,5'-tetra-t-butyl-4,4'-diphenoquinone can be reduced to TTBP by treatment with a reducing agent such as hydrogen. But this reduction reaction is usually performed after the 3,3',5,5'-tetra-t-butyl-4,4'-diphenoquinone has been isolated, so the overall manufacturing process will inevitably become complicated.
Japanese Pat. Application (OPI)No. 200935/1986 discloses a process for producing p,p'-biphenol which generally comprises: treating 2,6-di-t-butylphenol with oxygen or a like substance in the presence of a potassium hydroxide catalyst until 20-40% of the 2,6-di-t-butylphenol is converted to 3,3',5,5'-tetra-t-butyl-4,4'-diphenoquinone; reacting the residual 2,6-di-t-butylphenol with 3,3',5,5'-tetra-t-butyl-4,4'-diphenoquinone; distilling off the unreacted 2,6-di-t-butylphenol under vacuum and recovering the liquid bottoms chiefly composed of TTBP; dissolving the recovered liquid bottoms in a specified solvent; filtering off the potassium hydroxide catalyst; and debutylating the 3,3',5,5'-tetra-t-butyl-4,4'-dihydroxybiphenyl in the filtrate in the presence of a catalyst.
However, this method has the problem that in order to remove the potassium hydroxide catalyst from the reaction solution, the liquid bottoms chiefly composed of 3,3',5,5'-tetra-t-butyl-4,4'-dihydroxybiphenyl must be dissolved in a specified solvent such as diphenyl ether. As a further problem, the starting 2,6-di-t-butylphenol must be distilled off under vacuum in order to obtain the liquid bottoms. Because of these problems, the method still suffers a disadvantage in that it involves too complicated procedures to justify its application to commercial production of p,p'-biphenol.
As described above, p,p'-biphenol has been known to be capable of being produced from 2,6-di-t-butylphenol. Conventionally, the reaction expressed by scheme (1) is completed with proper conditions being selected with respect to such factors as catalyst, and thereafter TTBP is separated from the reaction mixture. Then, with another set of conditions including acid catalyst being selected, the second-stage reaction (dealkylation) expressed by scheme (2) is performed to produce p,p'-biphenol.
However, in order to produce p,p'-biphenol from 2,6-di-t-butylphenol by the method described above, TTBP must be separated from the mixture consisting 2,6-di-t-butylphenol, 3,3',5,5'-tetra-t-butyl-4,4'-diphenoquinone and TTBP, and this adds to the complexitity of the overall process or p,p'-biphenol production.